1. Field of the Invention
The present invention relates to a radio signal receiving apparatus and a radio signal receiving method, and more particularly to a radio signal receiving apparatus for and a radio signal receiving method of receiving a plurality of radio signals respectively having base band signal components through a plurality of antennas, and selectively outputting base band signals indicative of the base band signal components.
2. Description of the Related Art
Up until now, there have been proposed a wide variety of conventional radio signal receiving apparatus available for various kinds of wireless communications appliance such as for example a mobile phone for the purpose of ensuring reliable communications between a far-end terminal and a near-end terminal.
One typical example of the conventional radio signal receiving apparatus of this type is shown in FIG. 20. The conventional radio signal receiving apparatus 500 is shown in FIG. 20 as comprising first and second radio signal receiving circuits 501 and 502 for respectively receiving radio signals through antennas, and producing base band signals respectively indicative of the base band signal components of the radio signals, a base band signal selecting circuit 570 for selecting one of the base band signals from among the base band signals respectively produced by the first and second radio signal receiving circuits 501 and 502, and a base band signal outputting circuit 580 for outputting the base band signal selected by the base band signal selecting circuit 570 to an exterior device, not shown. The base band signal component is constituted by two different signal components consisting of I-axis and Q-axis signal components which are orthogonal with each other.
The first radio signal receiving circuit 501 includes a radio signal amplifier 511 for amplifying the radio signal, an analog-to-digital converter, hereinlater simply referred to as “AD converter”, 512 for converting the amplified radio signal received from the radio signal amplifier 511 into a digital radio signal, a demodulating and Nyquist filtering circuit 513 for demodulating the digital radio signal received from the AD converter 512 to produce an I-axis signal indicative of the I-axis signal component of the base band signal and a Q-axis signal indicative of the Q-axis signal component of the base band signal, a publicly-known Auto Gain Control circuit, hereinlater simply referred to as “AGC circuit”, 530 for controlling the radio signal amplifier 511 to have the radio signal amplifier 511 amplify the radio signal in response to the signal level of the base band signal to be produced by the demodulating and Nyquist filtering circuit 513, a base band signal producing circuit 551 for decoding the base band signal, and producing a base band signal having the base band signal component of the radio signal received from the demodulating and Nyquist filtering section 513, and a synchronization circuit 552 for controlling the base band signal producing circuit 551 to allow the base band signal produced by the base band signal producing circuit 551 to be synchronized with the base band signal produced by a base band signal producing circuit 561 forming part of the second radio signal receiving circuit 502. The constitutional elements of the first radio signal receiving circuit 502 are the same in construction as those of the second radio signal receiving circuit 501 as shown in FIG. 20.
The AGC circuit 530 is adapted to calculate a signal level value indicative of the signal level of the received radio signal to be amplified by the radio signal amplifier 511. The base band signal selecting circuit 570 is operative to select one radio signal receiving circuit from among the radio signal receiving circuits 501 and 502 after judging whether or not the signal level value calculated by the AGC circuit 530 is grater than the signal level value calculated by the AGC circuit 540. If it is judged that the signal level value calculated by the AGC circuit 530 is higher than the signal level value calculated by the AGC circuit 540, the base band signal selecting circuit 570 selects the radio signal receiving circuit 530 as the selected radio signal receiving circuit. The base band signal outputting circuit 580 is operative to receive the base band signal from the selected radio signal receiving circuit and output the base band signal thus received as the selected base band signal.
In the conventional radio signal receiving apparatus 500, the AGC circuit 530 is operated in accordance with a time constant inherent in the AGC circuit 530. This means that the AGC circuits 530 and 540 are designed to calculate and update the signal level values of the radio signals received from the radio signal receiving circuits 501 and 502 at time intervals defined by the time constant. This leads to the fact that the AGC circuit 530 of the radio signal receiving circuit 501, for example, calculating a signal level value higher than a signal level value calculated by the AGC circuit 540 of the radio signal receiving circuit 502 and being selected by the base band signal selecting circuit 570 at a time point tends to calculate a signal level value much lower than a signal level value calculated by the AGC circuit 540 of the radio signal receiving circuit 502 at a subsequent time point when the radio signal receiving circuit 501 thus selected actually outputs the base band signal to the base band signal outputting circuit 580.
The conventional radio signal receiving apparatus, in which the radio signal selecting circuit 570 is operative to select one radio signal receiving circuit from among the radio signal receiving circuits 501 and 502 in response to the signal level values calculated by the AGC circuits 530 and 540 at a time point, encounters a drawback that the conventional radio signal receiving apparatus tends to select the radio signal receiving circuit outputting the base band signals respectively having signal levels, which are monotonously decreased along a time axis, thereby aggravating the quality of communications between a far-end speaker and a near-end speaker.
Furthermore, the radio signals are subject to interferences such as, for example, a fading interference, and a multi-path interference, depending on an environment in which the radio signals propagate. The radio signal receiving circuits 501 and 502 tend to receive the radio signals deteriorated in quality due to the interference. The conventional radio signal receiving apparatus, in which the base band signal selecting circuit 570 is operative to select one radio signal receiving circuit from among the radio signal receiving circuits 501 and 502 in response to the signal level values calculated by the AGC sections 530 and 540 of the radio signal receiving circuits 501 and 502, encounters another drawback that the conventional radio signal receiving apparatus 500 may select the radio signal receiving circuit outputting the base band signal having errors caused by the interferences.